Method of operating a reactor during start-up or shut-down



Se t. 14, 1965 R. G. MQGRATH METHOD OF OPERATING A REACTOR DURINGSTART-UP OR SHUT-DOWN 5 Sheets-Sheet 1 Original Filed Sept. 2, 1958iNVENTOR Robert G. McGrufh A ORNEY Sept. 14, 1965 ca. M GRATH 3,206,371

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0 U4 U2 3/4 t Fraction of Full Power United States Patent 3,296,371METHOD OF OPERATING A REACTOR DURING START-UP 0R SHUT-DOWN Robert G.McGrath, Penn Hills Township, Allegheny County, Pa, assignor toWestinghouse Electric Corporation, Pittsburgh, Pa, a corporation ofPennsylvania Continuation of abandoned application Sen. No. 758,360,Sept. 2, 1958. This application Dec. 28, 1962, Ser. No. 248,155

4 Claims. (Cl. 17648) This application is a continuation of applicantscopending application of the same title filed September 2, 1958; SerialNo. 758,360, now abandoned.

The present invention relates to a method for operating a neutronicreactor, particularly during starting up or shutting down thereof.

In certain types of neutronic reactors, an example of which is describedhereinafter in greater detail, the reactor and the primary circulatingsystems associated therewith are arranged for adding or withdrawing, orotherwise supplying nuclear fuel or fissile material to the reactor invarying masses or concentration relative to the volume comprised withinthe reactional vessel. The quantity of fuel contained within thereactional vessel may be varied in a number of ways. For example, anumber of fuel rods containing fissile material can be inserted into orwithdrawn from the reactional vessel in the manner in which the controlrods of certain types of reactors are moved relative to the vessel, orthe fissile material might be conveyed in varying quantities to thereact-or vessel in a form of a fluidized solid or in a form of largerdiscrete particles either of which may be supported by a gas or othervehicular fluid. In still other neutronic reactor systems the fissilematerial can be supplied to the reactional vessel in the forms of ametallo-organic compound of the fissile material which is a liquid atreactor operating temperatures, a solution of a soluble fissile materialsuch as uranylsulfate, or as a suspension or slurry of desirably ultrafine particles of the fissile material or their oxides in a suitablevehicle such as that described hereinafter. Although the inventiondisclosed herein is adapted for use in operating many of theaforementioned variable concentration-type reactors, the invention willbe described in greater detail in connection with a quasi-homogeneous,slurry-type reactor.

It has been found that, in certain neutronic reactors wherein exhibitinga negative temperature coelficient and the concentration of the fuel canbe varied readily, there are two concentrations of the fuel whereat theneutronic reactor can maintain criticality (K =1) at a given operatingtemperature which is less than the maximum workable temperatureobtainable by the reactor. These chain-reacting concentrations aredesignated as respectively the high and low critical concentrations orpoints. However, in the range of fuel concentrations between thesecritical concentrations, the reactor would become supercritical (K l)relative to the aforesaid operating temperature; and the temperaturewould increase markedly, if no means wer employed to control the reactoruntil K would again become unity or less, provided the reactor systemcould withstand the added thermal strains. Obviously it is desirable tooperate the reactor system at an operating temperature less than themaximum workable temperature to reduce such thermal strains to allowablemargins of safety. Moreover, it is desirable to operate the neutronicreactor at the high critical concentration corresponding to theaforesaid operating temperature for the reason that, with the addedfissile material thereby included within the reactor vessel, a greaterneutronic economy is obtained; and hence a greater conversion ratio ofthe fertile isotope, usually included in the fissile material,

ice

to one of the fissionable isotopes is likewise secured. It has likewisebeen found that when operating an efficient neutronic reactor at thehigh critical concentration that conversion ratios of unity or greatercan be obtained, that is to say, that at least as much fissionableisotope can be transmuted from the fertile material as is consumed inthe chain reaction sustained in the intial supply of fissionableisotope. The basic mechanisms whereby the fertile isotopes are convertedinto the corresponding fissionable isotopes in a neutronic reactor aredescribed hereinafter in greater detail.

As pointed out previously, however, the average temperature of thereactor rises to a high peak at a certain fuel concentration betweengiven high and low critical points, as the concentration is raised fromthe low critical point to the corresponding high critical point of agiven operating temperature, if all other factors remain the same. Thiscondition results in the aforementioned supercritical condition whereinthe effective constant of criticality (K would become slightly greaterthan unity for the range of concentrations between these criticalpoints, if the temperature were to remain constant. The temperatureupswing in certain cases, however, can be sufficiently high, ifuncontrolled, to exceed the design limitations of the reactor and theassociated primary equipment.

In spite of the foregoing remarks, it has been found that the safestmethod of adding fissile material to the reactor is to fill the primaryreactor system initially with either a dilute mixture of fissilematerial and a suitable vehicle or with the vehicle alone. The dilutemixture, if employed, is maintained at a concentration which issufficiently low to preclude criticality under any conditions. Theconcentration of fissile material then is gradually increased throughthe low critical concentration to the high critical concentration oroperating concentration.

As the concentration is increased between the low and high criticalconcentrations, suitable means are employed to control the reactor inthe area of supercriticality between these concentrations. Desirably,the reactor is operated for a time at the low critical concentration inorder to produce sufiicient internal Xe to shut down the reactor whilethe concentration is being increased from the low to the high criticalconcentration. This method of operating the reactor is described fullyand claimed in a copending application of William A. Frederick, entitledMethod of Operating a Nuclear Reactor, filed December 20, 1957, S. N.704,098, now US. Patent 3,155,596, dated November 3, 1964, and assignedto the present assignee. Alternatively, an external reactor poison, forexample a boronic compounds, can be added to the primary system inaccordance with known methods; or on the other hand the primary reactorsystem can be designed to withstand the higher temperature necessary toshut down the reactor due to the negative coefiicient of reactivityexhibited by thermal and epithermal reactors and described hereinafter.

It may be suggested that the reactor system be filled initially withfissile material at a concentration above the high criticalconcentration, which is then gradually reduced to the operatingconcentration. This method is not feasible due to the possibility of aportion of fissile material settling out of the vehicle after additionto the reactor, particularly before the circulating pumps can bestarted, and to the attendant great danger of unpredictable oriticalityin the fiissile material when thus diluted. Moreover, the fissilematerial would have to be maintained at an increasingly high temperatureas the concentration is reduced in order to preclude prematurecriticality. For practical purposes, then, the reactor system would haveto be raised to operating temperature by means of an external source ofheat before beginning to decrease the concentration of fissile materialin order to preclude the possibility of thermal shock.

It may be suggested also that the primary reactor system be filledinitially with fissile material at the operating or high criticalconcentration. This method likewise is inappropriate due tounpredictable reactivity of the fissile material while being added inthis concentration. Both the reactor system and fissile material wouldhave to be preheated -in some fashion to a temperature in excess ofoperating temperature to prevent premature criticality and thermalstresses in the reactor system. Moreover, when employing certain typesof vehicles for the fissile material, these vehicles must be maintainedunder considerable pressures to prevent boiling at the operating reactortemperatures. However, the required pressurizing is impractical untilthe reactor system, including usually a reactional vessel and a numberof circulating or cooling loops, is completely filled. Furthermore, theprimary circulating purnps cannot be started until the reactor system iscompletely filled, if the fissile material is in liquid form, because ofvapor binding. Consequently, a portion of the fissile material maysettle out of the vehicle, whereupon the concentration of the remainingfissile material may fall into the aforementioned supercritical areabetween the low and high critical concentrations.

Specifically, the invention relates to an apparatus and method for usein starting up and shutting down a neutronic reactor so that the reactorcan be initially filled either with a very dilute mixture of fissilematerial in a suitable vehicle or with the vehicle alone. Subsequently,the concentration of the fissile material is gradually increased bymeans provided in accordance with the invention until the desiredoperating concentration is attained. In shutting down the reactor thereverse procedure is followed. The aforementioned apparatus is arrangedso that the rate of increase or decrease in fissile concentration withinthe primary reactor system can be readily changed or otherwisecontrolled in order to .avoid large inputs of reactivity in the primaryreactor system. Although the invention is described hereinafter inconjunction with a slurry type homogeneous reactor having high and lowcritical fuel concentrations, it will be obvious as this descriptionproceeds that the invention is not limitedthereto.

In view of the foregoing it is an object of the invention, to provide anovelv and efiicient neutronic reactor system particularly of the typein which the fuel concentration thereof can be readily varied.

Another object of theinvention is to provide novel apparatus adapted foruse with a neutronic reactor for the purpose of facilitating filling anddraining the primary system thereof.

Still another object of the invention is to provide a novel method andapparatus of the character described r for varying the fuelconcentration of a homogeneous or quasi-homogeneous type reactor in acarefully controllable manner.

A further object is the provision of apparatus of the characterdescribed which is adapted for coupling to the primary circulatingsystem of a reactor and which is operable without the use of additionalpumping means.

These and other objects, features and advantages of the invention willbe made apparent during the forth coming description of an illustrativeembodiment of the invention with the description being taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic and elevational view, partially sectioned, of anyexemplary homogeneous-type reactional vessel shown in conjunction withprimary coolant loop circuitry;

'FIGS. 2A, 2B, 2C constitute a schematic fluid 'circuit diagram of ahomogeneous reactor system such as that illustrated more generally inFIG. 1 and arranged for use with certain auxiliary equipment;

FIG. 3 is a graph showing a series of temperature curves of theeffective neutronic multiplication constant plotted against slurryconcentration; and

FIG. 4 is a graphical representation of slurry and steam temperatures ofthe reactor system described herein, as functions of reactor poweroutput.

Generally speaking, in a homogeneous-type reactor system, the nuclearfuel is contained within the system as a liquid or suspension, which insome cases may be a liquid compound of at least one of the fissileisotopes noted below. In other cases, the liquid fuel comprises asuspension in a suitable vehicle of a pulverulent form of one or more ofthese fissionable and fertile isotopes, or a combination thereof, or asolution of one or more cmpounds thereof in a suitable solvent such asWater. In those systems wherein the fuel is employed as a conventionalsuspension or slurry or in some other fluidized form wherein the fuel ispresent in discrete, movable particles, the reactor system is sometimesdesignated as quasi-homogeneous. As explained more thoroughlyhereinafter, the fluidized fuel is circulated through a reactor vesselby one or more primary circulating loops provided with suitable pumpingmeans. The fluid fuel including the vehicle or solvent, which usuallyserves both as coolant and moderator, thus circulates through both thevessel and the circulating loops in contradistinction to a heterogeneoustype reactor system. In the latter class of reactors the fuel,moderator, and the coolant or coolant-moderator usually are physicallyseparated and at least the fuel is mounted fixedly and entirely withinthe reactional vessel.

The homogeneous reactional vessel is fabricated of such size and shapethat a quantity of the circulating fluid fuel contained therein isequivalent to the critical mass of the chain-reacting isotope includedin the fuel and consequently a self-sustaining chain reaction can beestablished in the vessel. In the case of a quasi-homogeneous reactor,the concentration of the fissionable or chain-reacting isotope in theslurry or suspension can be adjusted within rather wide limits such thatthe aforesaid size and shape of the vessel can be varied accordingly asdesired. As pointed out hereinafter, the remaining components of thesystem are insufficient in size and are suitably spaced or shielded suchthat a critical mass cannot be accumulated elsewhere in the reactorsystem. The heat developed within the circulating fuel as a result ofthe nuclear chain reaction is removed from the fuel as it circulatesthrough the primary loops by suitable heat exchanging means coupledwithin each of these loops.

The vehicle or solvent employed with the circulating fuel, which may beordinary water (H O), heavy water (D 0) or an organic material havingthe desired characteristics of temperature and radiation stability,serves as a moderator for the chain reaction in addition to serving as aheat transfer medium as noted heretofore. As is well known, a moderatormaterial usually is employed adjacent the nuclear fuel to slow the fastneutrons produced by each fission to thermal velocity, whereat theneutrons are most efiicient for inducing fission in atoms comprising thefissionable isotopes. More specifically, the moderator material slowsneutrons having energies in the neighborhood of ten million electronvolts to energies which are equivalent to thermally excited hydrogenatoms or about 0.1 electron volt. As a result, the moderator materialappropriately is selected from a material having the characteristics oflow neutronic capture crosssection and a high neutronic scatteringcross-section. Suitable materials for these purposes include carbon andthe vehicles or solvents noted heretofore, i.e., light and heavy water,and hydrocarbon organic materials which, of course, contain carbon andhydrogen.

The homogeneous reactor system, presently to be described, is controlledinherently by the negative temperature coefiicient of reactivityassociated with the circulating nuclear fuel. This phenomenon iscomparatively reactor system.

Well known and is based upon the fact that an increase in temperature ofthe fuel contained within the reactor vessel decreases the density ofboth the fuel and the vehicular moderator and likewise its moderatingcharacteristics. By the same token, this decrease in density increasesthe number of neutrons which are lost from the periphery of thechain-reacting mass, and the resulting loss in neutron economy decreasesthe reactivity of the Additional control is accomplished, as required,by diluting the circulating fuel with additional vehicle or solvent, byadding a neutron absorbing poison such as cadmium, boron, or xenon, orby draining the contents of the reactional vessel into a series ofstorage tanks presently to be described. The latter arrangement alsoserves to terminate the chain reaction completely in an emergency or toshut down the reactor for maintenance and the like.

The fissional products which are formed during operation of the reactordesirably are extracted continually from the system by means of chemicalprocessing in the case of solids, or in the case of gases, by means ofan off-gas system such as that described in copending applications of D.Rinald and of J. Weisman et al., filed October 21, 1957, Serial No.691,264 and 691,263, now Patents 3,080,307, dated March 5, 1963, and3,093,564, dated June 11, 1963, respectively, and both assigned to thepresent assignee. These fissional products cannot be permitted toaccumulate within the reactor system during normal operation thereofinasmuch as some of the daughter isotopes, particularly xenon 135,quickly terminate or poison the chain reaction although present inrelatively small concentrations. As pointed out hereinafter in greaterdetail, use can be made of this fact in controlling and opertaing avariable fuel concentration type reactor. In any event, the accumulationof these isotopes which result either directly, or indirectly throughradioactive decay, from the fissional process would tend to increaseradioactivity assoicated with the reactor plant as compared to theconditions obtaining were the fissional products continuously removed.As a result, the normal biological shielding requirements for thereactional vessel, the fuel circulating loops, and associated equipmentwould be increased. Moreover, many of the longer-lived, fission-producedisotopes are valuable per se for those research, industrial, andmedicinal applications, which require high levels of the variousradioactive emanations.

The circulating nuclear fuel in a simple burner type homogeneousreactor, contains a high percentage of one or more of the knownfissionable isotopes U U Pu amounting of course, to a quantitysufficient to sustain a chain reaction. Although a simple burner type ofreactor is relatively more efiicient as to size and neutron economy, itsoverall fuel cost is very high. Moreover, substantially no additionalfuel is produced during operation of this type of nuclear reactor. Onthe other hand, in regenerative or breeder types of homogeneousreactors,.an additional quantity of a fertile isotope such as 90Th or Uis admixed uniformly with the circulating fuel material. Thelatter-mentioned fertile .isotope can be supplied in the form of naturalor source grade uranium which is primarily the U isotope containingapproximately 0.7% of U In a heterogeneous type reactor, the samecombinations of fissionable and fertile isotopes can be employed, withthe exception that both groups of the fissile isotopes are fixedlymounted within the reactor core and that the fertile isotope, commonlyreferred to as blanket material, usually surrounds the fissionablematerial. However, in a uniform, low enrichment heterogeneous reactor,several designs of which are either extant or under consideration, thesocalled blanket or fertile material, of course, is mixed uniformly withthe fissionable isotope. In the latter class of reactors, U usually isemployed which has been enriched to a greater than natural percentage ofU In an eflicient reactor of the previously-mentioned regenera- 6 tivetypes, it is possible to generate-from the one or more fertile isotopesat least as much fissionable isotopes as is consumed in the chainreaction. If the conversional ratio is greater than unity, the reactoris classified in the breeder category.

During the progress of the chain reaction, each fissioned atom emits anaverage of two to three neutrons, some of which are classified as fastneutrons and must be slowed to thermal energies as noted previously.Approximately one of these neutrons is utilited in propagating the chainreaction. Another one of the neutrons is employed to initiate one of theseries of nuclear reactions described below, whereby an atom of thefertile or blanket material is transmuted into an atom of fissfonableisotope, and the amount thereof may be equivalent, for example, to theamount of fissionable material consumed in the chain reaction. If suchis the case, only the fertile material need be added to the reactorsystem during its operation. The remainder of the fissionproducedneutrons are absorbed in structural and moderator materials, innon-fissioning capture of atoms of fissile material, and in peripheralescape from the chain-reacting mass.

Upon capturing one of the aforesaid fissional neutrons the fertilematerial U if employed, is converted into an isotope of the transuranicelement plutonium Pu in accordance with the following nuclear equations:

23 min. 2.3d.94

23 min. The resultant fissionable isotope U having a half-life of163,000 years, likewise is relatively stable.

Referring now more specifically to FIG. 1 of the drawings, an exemplaryvariable fuel concentration type of reactor system is disclosed, whichis adapted for operation in accordance with the invention. In thisexample, the reaction system is a quasi-homogeneous, or slurrytypesystem, and comprises a reactional vessel 20 having a spheroidalconfiguration and provided at diametrically opposite areas thereof withinlet and outlet manifolds 22 and 24, respectively. The reactionalvessel 20 is of suflicient size to contain, as aforesaid, a criticalmass of the circulating nuclear fuel flowing through the vessel and theprimary loops of the reactor system. In this application, wherein acirculating slurry containing suspended, uniformly, admixed, pulverulentoxides of thorium (T1102) and highly enriched uranium U0 is employed,with a vehicle including deuterium oxide or heavy water (D 0), theinside diameter of the innermost reactor vessel thermal shield 40 is ofthe order of 13 feet. The aforementioned slurry, which is describedsubsequently in greater detail, thus includes a fissionable material inthe form of uranium 235 and a fertile material, thorium 232.Additionally, a small proportion of the fertile material, uranium 238,is included unavoidably with the U isotope.

In this example of the homogeneous reactor system a total of fourcirculating loops, with only one loop 25 being shown in FIGS. 1 and 2,are connected to the intake and outlet manifolds 22 and 24 by means ofinlet and outlet conduits 26 and 28, respectively. The outlet conduit 28is connected to a gas separator 30 which in turn is coupled in serieswith a steam generating heat exchanger 32 coupled through a conduit 35to the suctional side 34 of a primary slurry pump 36. The gas separator30 is designed in a conventional manner and is arranged to removefissional and radiolytic gases from the system, which gases areconducted out of the separator by means of a conduit 31. The steamgenerator 32 which is provided, inter alia, with a feed water inlet 33and a steam outlet conduit 37 is constructed similarly to that describedin a copending application of William A. Webb et al., entitled RemoteEquipment Maintenance, Serial No. 659,002, filed May 15, 1957, assignedto the present assignee and now abandoned. The discharge side of thepump 36 is coupled to the inlet conduit 26 and manifold 22 of thereactor vessel.

In this example, the reactor vessel 20 is formed from a plurality ofspheroidal sections 38 which are welded together as shown to form thecompleted vessel. In order to minimize neutron-induced thermal stresseswithin the walls of the vessels 20, which are of the order of six andone-half inches in thickness, a plurality of thermal shields, indicatedgenerally by the reference character 40, are disposed adjacent the innersurface of the reactor vessel walls. The thermal shields 40 conformgenerally to the inner configuration of the vessel walls and are spacedtherefrom and from one another in order to provide, in this example,flow channels therebetween for the passage of a peripheral portion ofthe circulating nuclear fuel. Inasmuch as the thermal shields 40 aresubjected to little or no pressure differentials, they are maderelatively thin with respect to the vessel walls 20. A plurality ofbafiies 42 are disposed adjacent the lower or intake manifold 22 and aresuitably shaped for distributing the incoming slurry as indicated byflow arrows 44 throughout the interior areas of the vessel 20 and fordiverting a peripheral portion of this flow through the passages formedbetween the thermal shields 40 and adjacent the inner wall of the vessel20. A neutron reflecting member (not shown) can be disposed adjacent thethermal shields to reflect peripheral neutrons back into the centralregion of the vessel 20 in order to improve the neutron economy of thechain reaction.

The disposition of the thermal shields 40 in this manner substantiallyprevents impingement of fission-neutrons upon the adjacent vessel walls.Accordingly, the heating effect of the impinging neutrons is developedalmost entirely within the thermal shields 40 which are not subject topressure stresses as are the walls of the pressurized vessel 20.Moreover, the heat developed within the thermal shields 40 is readilyremoved by the peripheral portion of the circulating fuel flowingthrough the channels therebetween. Alternatively, the thermal shields 40can be replaced by the shield arrangement disclosed and claimed in acopending application of W. P. Haass, entitled Reactional Vessel, SerialNo. 652,627, filed April 12, 1957, now US. Patent 3,075,909, datedJanuary 29, 1963, and assigned to the present assignee.

The pressurized reactional vessel 20 is mounted upon an annularsupporting collar indicated generally by the reference character 46 andmounted upon a biological shielding wall portion or support 48. Thismounting arrangement for the reactor vessel 20 and the physicaldistribution of the primary circulating loops and other equipmentassociated therewith are described in greater detail in a copendingapplication of W. A. Webb et al., entitled Shielded Reactor PlantArrangement and Personnel Access Means Therefor, Serial No. 659,004,filed May 14, 1957, now US. Patent 3,113,915, dated December 10, 1963,and assigned to the assignee of the present invention.

In order to drain the contents of the reactional vessel, a drain outlet50 disposed in the lower or intake manifold 22 is coupled to a series ofslurry drain tanks 52, through a conduit 54. When it is desired to fillthe reactor system, the slurry contained in the drain tanks 52 is tratedschematically therein.

returned through another conduit 56 which is coupled to one or more ofthe circulating loop conduits 35. To aid in filling the reactionalvessel and associated loops, an auxiliary slurry pump 58 is coupled intothe conduit 56. The physical disposition of the drain tanks 52 relativeto the nuclear power plant arrangement is described in greater detail inthe last-mentioned copending application. For the present, it may bepointed out that the drain tanks 52 are provided in suflicient number tocontain at least all of the circulating nuclear fuel slurry of thesystem but are of such size that none of the tanks can contain acritical mass of slurry. Suitable neutronabsorbing material (not shown)is disposed between adjacent tanks in order to prevent the developmentof a chain reaction within the collective group of tanks when they arefilled with the circulating fuel.

In one exemplary arrangement, the fluid fuel contained within each ofthe drain tanks 52 is stirred constantly by individual agitators andstirrers 59 mounted adjacent the top of each of the tanks 52. The tanks52 and the agitators 59 desirably are hermetically sealed to preventleakage of biologically hazardous fluid and desirably are provided inthe form of that disclosed and claimed in a copending application of Meiand Widmer, entitled Sealed Agitator, Serial No. 672,661, filed July 18,1957, and now US. Patent 2,907,552 assigned to the present assignee.

The upper or outlet header 24 is fitted with an additional port 60 towhich a surge tank 62 is coupled by means of a conduit 66. In one formof homogeneous reactor system, the surge tank 62 comprises a relativelylarge volume which, however, is insutficient to contain a critical massof the circulating fuel. When in operation, a vapor space 68 is formedin the surge tank, which conveniently contains a vapor of the vehicleemployed in suspending the aforementioned fissionable and fertileoxides. As a result, during a positive system transient within thehomogeneous reactor system, a surge of liquid into the tank 62compresses the vapor confined within the surge tank space 63, therebyrelieving at least partially the increased pressures developed withinthe system.

A pressurizing vessel 64, which is coupled to the surge tank 62 by aconduit 67 connecting the vapor spaces thereof, is furnished with anumber of heating elements, indicated generally by the referencecharacters 70 and arranged for heating a portion of liquid, desirablythe same as the aforementioned liquid vehicle of the system. Thus, thereactor system is maintained at the desired operating pressure, byvaporization of the aforesaid vehicle portion. As a corollary functionthe pressurizing vessel 64 operates to maintain the aforementioned vaporspace or surge volume 68 within the surge tank 62. The pressurizingvessel 64 is provided with an inlet conduit 72 whereby the vessel 64 isinitially charged with the aforesaid vehicle portion and make-up vehicleis added to the pressurizing vessel as required.

Alternatively, the pressurizing vessel 64 and the surge tank 62 can bereplaced by the pressure regulating system claimed and disclosed in acopending application of Jules Wainrib, entitled Pressure ControllingSystem, Serial No. 677,942, filed August 13, 1957, now US.

Patent 3,060,110, dated October 23, 1962, and assigned.

to the present assignee.

Referring now more particularly to FIG. 2 of the drawings, the variousauxiliary equipment associated with the aforedescribed homogeneousreactor system, is illus- In the arrangement of the homogeneous reactorsystem, illustrated in FIG. 2, the primary slurry pump 36 is furnishedwith a capacity of approximately 8,000 gallons per minute which inconjunction with three other primary slurry pumps (not shown) disposedin a like number of similar circulating loop systems indicated generallyby arrows 25, produces a total rate of flow of approximately 32,000gallons per minute. Inasmuch as the reactor vessel 20 and thecirculating loops together enclose a total volume of approximately19,000 gallons, the circulating fuel is recycled through the system inabout one-half minute.

In this application of the invention, the circulating slurry comprises avehicle of deuterium oxide (D in which is suspended about 300 grams ofthorium oxide (ThO per kilogram of D 0 and approximately ten grams ofuranium oxide (U0 per kilogram of D 0. The uranium in this example isfully enriched and contains upwards of 90% of U isotope. Added with theuranium oxide is a very small proportion of a palladium catalystemployed to promote internal recombination of the major proportion ofthe radiolytic vehicular gases, deuterium and oxygen. The uncombined orremaining radiolytic gases are employed to sweep fissional product gasesout of the system, as explained hereinafter. The quantity of palladiumcatalyst, which is added in the form of the oxide (PdO) is of the orderof 0.001 gram per liter of slurry and can be replaced, if desired, byother suitable catalysts, for example, a platinum compound.

Accordingly, the system circulates a mixed oxide slurry with 'a totaloxide concentration in excess of 300 grams per kilogram of D 0 whichcorresponds to a solids content of about 3% by volume. The reactionalvessel and the circulating loops are maintained under a pressure in theneighborhood of 2,000 pounds per square inch absolute by operation ofthe pressurizing vessel 64. The pressurizing vessel 64, which desirablycontains only deuterium oxide or other such vehicle employed in thehomogeneous system as noted heretofore, is separated from the liquid orslurry portion of the surge tank 62 by means of the steam space 68thereof, to which the conduit 67 is coupled, thus avoiding the cakingthat would result if the circulating slurry itself were boiled in thepressurizing vessel 64.

Leaving the reactional vessel 20 the slurry stream branches into fourparallel identical circulating loops 25 only one of which is illustratedin detail. If desired, each loop can be isolated from the reactor by apair of dual stop valves 328 (FIG. 2) to permit certain types of remoteor semi-direct maintenance, without shutting down the entire plant, tobe performed on one of the circulating loops, for example, in the mannerdescribed in the copending, coassigned applications of Huston et al.,Serial No. 659,003, entitled Semidirect Equipment Maintenance, filed May14, 1957, now US. Patent 3,090,740, dated May 21, 1963, and of Webb etal., Serial No. 659,- 002 entitled Remote Equipment Maintenance, filedMay 14, 1957, and now abandoned.

Within the reactor vessel 20 part of the kinetic energy of the fissionalfragments is absorbed by the deuterium oxide molecules, some of whichare disassociat-ed into deuterium and oxygen gases. For the most partthese radiolytic gases are recombined within the reactor system throughusage of the palladium catalyst noted above. However, the remainingportion of these radiolytic gases is removed together with certaingaseous fission products by means of the gas separatous and conveyedthrough the conduit 31 to an external recombining unit associated with asuitable gas handling system (not shown). Suitable forms of gas handlingsystems adapted for recombining the radiolytic gases and for separatingand eliminating the fissioned product gases are disclosed and claimed inPatents 3,080,307 and 3,093,564, above referred to.

From the gas separators 30 the circulating slurry in each loop 25 isconducted to the respective steam generators 32, as noted heretofore.The steam developed in the steam generators is conducted through theoutlet conduits 37 and conduits 80 to suitable steam utilizing means,for example, one or more turbo-electric generators (not shown). Inaddition, a small portion of the steam is removed from one of the steamgenerators 32 and returned thereto by means of conduits 82 for thepurpose of supplying heat to various components of the primary reactorsystem. A plurality of storage tanks 84, with two being shown forpurposes of illustration, are coupled to the steam generators by meansof a conduit 86 for purposes of draining the steam side of the steamgenerators in the event of leakage of radioactive slurry into the steamside of the generators or other defect. Any vapor contained at this timewithin the steam generators is removed through overhead conduit 88 to asteam generator drain tank condenser 90. This vapor after beingcondensed in the condenser 90 is conducted to the aforementioned draintanks 84 through a conduit system 92. In the event that the contents ofthe steam generator drain tanks 84 become radioactive the contents canbe conveyed through the valved conduit system 94 to suitable means forconcentrating or otherwise preparing the radioactive material forunderground or oceanic burial.

As indicated heretofore, four circulating loops indicated generally bythe reference character 25, are associated with the reactor vessel 20;however, in the drawings only one of these loops are shown in detail,inasmuch as in this example these loops are substantially identical.However, as will be described subsequently in greater detail, the pipingconnections associated with one of the circulating loops 25 difiersslightly in that certain auxiliary equipment associated with the reactorsys tem are coupled to only one of the circulating loops 25. In the caseof the reactor system described herein, the most important of theseauxiliary systems is the fuel handling system which is illustrated indetail in FIG. 2 of the drawings. In this arrangement, the operationsperformed within the fuel handling system are divided into four majorcategories of slurry storage, vehicular deuterium oxide storage,concentration and dilution of the slurry, and transfer of slurry orvehicle into and out of the primary system.

In this arrangement the fuel handling system'comprises, inter alia,twenty drain tanks 52, although only five of these tanks are illustratedin FIG. 2. For purposes of exemplifying the invention the drain tanks52' are grouped into three functional categories in which a single draintank 52A serves as a slurry aaccumulator tank. A total of six draintanks 52B serve as storage for con centrated slurry and the remainingthirteen tanks 52C serve as a repository for the slurry normallycirculated through the reactor vessel 20 and the primary coolant loops25. The latter group of tanks 52'C are capable of containing the entirecontents of the primary reactor system or about 19,000 gallons and arenormally empty during reactor operation in the event the reactor systemmust be shut down under emergency conditions or for purposes ofmaintenance or other contingency. The aforementioned drain tankgroupings together with the common header conduits 96, 110, 144, and 182are sometimes hereinafter referred to as the drain tank complex.

Each of these storage tanks 52' is provided with a stirring mechanism59, which has been described previously in connection with FIG. 1 of thedrawings. As indicated heretofore none of the generally vertical draintanks 52 contains sufiicient volume to provide a critical mass of thefissile material contained therewithin. Moreover, the twenty drain tanks52 are arranged in a separated or spaced array and desirably areprovided with neutron absorbing material therebetween in order toprevent a critical mass from being formed collectively among the draintanks 52'. A suitable spatial disposition of the drain tanks 52 isdescribed in aforementioned US. Patent 3,113,915.

In this example, the drain tanks 52' are each approximately three feetin diameter and about thirty feet in height, which dimensions positivelypreclude criticality in the slurry contained within each tank under anyconditions. Each tank is designed to withstand an operating 1 1 pressureof 1500 psi. For purposes elaborated upon subsequently, the concentratedslurry contained within the drain tanks 52B in this example isapproximately double the normal concentration of the slurry circulatedthrough the primary reactor system.

The drain tank 52A serves as aforesaid, as a slurry accumulator tanknormally used to collect small volumes of slurry periodically dischargedor blown down from various components of the reactor system forcleansing purposes. All the tanks 52' are coupled to a drain header orconduit 96 through individual valved conduits 98. The aforedescribedgroupings 52A, 52B and 52C of the drain tanks 52' are preserved byinsertion of normally closed valves 100, 108 and 102 within the drainheader 96.

The reactional vessel 20 is coupled to the drain header 96 by means of aconduit 54 connected to the outlet port 50 of the lower reactionalvessel manifold 22 as described heretofore in connection with FIG. 1 ofthe drawings. To ensure quick and complete draining of each primary loop25, the suction side of each primary pump 36 and the inlet of each steamgenerator 32 are connected through a branched conduit system 104 to thedrain header 96. To control draining of the reactor system in thismanner a pair of stop valves 106 are inserted in each branch of theconduit 104 and in the conduit 54. In this manner the reactioned vessel20 and each of the four circulating loops 25 are coupled through thedrain header 96, a normally opened valve 108 in the drain header 96, andthe drain tank inlet conduits 98C to the thirteen slurry repositorytanks 52'C.

By slidably opening one or more of the valves 100, 102, or 108 in thedrain header conduit 96 the slurry accumulator tank 52'A in an emergencycan be utilized either with the group of six concentrated slurry tanks52,B or with the group of thirteen slurry repository tanks 52C.

Thus, it will be seen that the drain header mainly serves to connect atotal of nine points of drainage i.e., one at the reactor vessel 20itself and two at each of the primary loops 25, from the primary reactorsystem to the drain tanks 52'. Among the auxiliary functions of thedrain header 96 are transfer of deuterium oxide steam from a suitableevaporator (not shown) to the drain.

tanks 52 for heating and pressurizing the drain tanks 52 to preventthermal shock upon contact by hot slurry drained from the primaryreactor system, return of condensed deuterium oxide from a plurality ofcondensers 114 and 115 presently to be described to the drain tanks 52,and transfer of material from one drain tank to the drain header 96 orto another in the manner to be presently described.

Each of the twenty drain tanks 52' also is coupled to a liquid header orconduit 110 by means of individual valved conduits 112. If desired, asmall portion of slurry can be drawn periodically from the drain tanks52 and conducted by means of the liquid header conduit 110 and a conduit116 to a chemical processing plant 118. At the processing plant 118 thisportion of slurry is chemically processed to remove fissional productscreated during normal reactor operation. The reprocessed slurry of thechemical processing plant 118 can be conveyed to the primary reactorsystem by means of a slurry pump 318 and valved conduits 120 and 121.The conduits 120 and 121 conduct the reprocessed slurry to a battery ofhigh head pumps, indicated generally by the reference characters 122,and thence to one of the circulating loops 25 by way of a valved conduit124 and one of the branched conduits 104 described heretofore. When notbeing added to the primary reactor system in this fashion, the output ofthe chemical processing plant 118 can be conveyed by means of the slurrypump 31% through a suitable cooler 126, conduits 121 and 128, and avalve conduit 130 to the liquid header 110. 1 Alternatively, as in thisarrangement 12 when the reactor is not in operation, the output of thechemical processing plant can be conveyed through the cooler 126 andconduit 128 as before, and through another valved conduit 132 to theslurry accumulator tank 52A.

However, when filling the primary reactor system including the vessel 20and associated circulating loops 25, slurry is withdrawn from the draintanks 52 in the reverse direction through the valved conduit 130 or 132or both, and through the cooler 126 and the associated conduit 128 bymeans of the aforesaid battery of high head pumps 122 and the conduits120 and 124. As pointed out previously, the conduit 124 is coupled toone only of the circulating loops 25 through the associated one of thebranched conduits 104. The liquid header 110 is provided with valves 134and 136, which in conjunction With a valve 138 disposed in the conduitconnection 112A of the slurry accumulator tank, determined which of thethree groups of storage tanks 52A, 52B or 52C are coupled to the suctionside 140 of the high head pumps 122.

After the reactor system is filled, the primary function of the liquidheader 110 is to provide for over-flow from one storage tank 52' toanother in each group and to assure level equalization among the draintanks in each group. The liquid header also is employed to transferslurry vehicle, which in this case is deuterium oxide, to one or more ofthe drain tanks 52 for purposes of dilution or the like, to transferslurry as subsequently described from one tank to another, or to conveyslurry from one or more of the drain tanks to the chemical processingplant 118 in the manner described heretofore. The individual conduitconnections 112 which couple the storage tanks 52 to the liquid header110 extend to the bottom of each tank to permit almost complete liquidremoval therefrom. These extensions of the conduits 112 are indicated bydashed lines 142.

After the slurry has been subjected to chain reaction within thereactional vessel 20 and subsequently stored in the drain tanks 52, theslurry will release a considerable amount of heat due to radioactivedecay of the contained fission products. The decay heat is removed fromthe storage tanks 52 by condensation of that portion of the deuteriumoxide vehicle which is vaporized by the decay heat. In furtherance ofthis purpose, each of the drain tanks 52' is coupled to a vapor header144 through individual vapor conduit connections 146. That portion ofthe vapor header 144 which is coupled to the slurry repository tanks SZCis isolated from the remainder of the vapor header 144 by a pair of stopvalves 148. By the same token that portion of the vapor header coupledto the slurry accumulator tank 52'A can be isolated from the remainderof the vapor header 144, if desired, by means of a normally open valve150.

More specifically, any heat developed in the slurry accumulator tank52'A and in the six concentrated slurry tanks 52B is removed byconveying the resultant deuterium oxide vapor from these tanks 52A and52B through the associated valved conduit connections 146, the vaporheader 144, and a valved inlet conduit 152, to a slurry entrainmentseparator 154. The slurry entrainment separator 154 is a conventional,centrifugaltype device and therefore will not be described in detail.The entrained liquid output of the entrainment separator is returned tothe drain header 96 or to the liquid header 110 through a conduit 156and valved conduits 158 and 160, respectively. Thus it is seen that theentrained slurry can be returned to the liquid header 110 and thence toone or more of the storage tanks 52A or 52B by opening an appropriateone of valves 162 or 164 of the aforesaid conduits 158 or 160.

The vapor from which any entrained slurry has been removed is thenconveyed to an associated drain tank condenser 114 through a conduit166. After being condensed the liquid deuterium oxide is conveyed fromthe condenser 114 through a valved outlet conduit 168 and the valvedconduit 158 or 160 described heretofore to either the drain header 96 orthe liquid header 110. A check valve 170 is disposed in the valvedconduit 168 in order to prevent reverse flow of slurry from theentrainment separator 154 which slurry would tend to clog the heatedportions of the condenser 114.

In a similar fashion the vapor removed from the slurry repository tanks52C and the associated portion of the vapor header 144 is conducted inparallel paths through valved conduits 171 to a pair of slurryentrainment separators 172. The effluent vapors of the slurryentrainment separators 172 are conducted to associated ones of the draintank condensers 115. The outputs of the drain tank condensers 115 and ofthe slurry separators 172 are conveyed respectively through conduits 174and 176 and one of the valved conduits 178 or 180 to either the drainheader 96 or the liquid header 110, as desired. As stated heretofore thevapor generated in the repository tanks 52C and conducted through thevapor header 114 is normally isolated from the balance of the vaporheader by means of the stop valves 148. Thus, it will be seen that twodrain tank condensers 115 and associated components are reserved for usewith the repository tanks 52C while one condenser 114 and entrainmentseparator 154 are employed with the balance of the drain tanks. Thisarrangement is necessary due to the larger number of repository tanks52C and to the fact that these tankes normally are intially employed tostore very hot slurry, which moreover contains a quantity of fissionproducts as a result of having been subject to the fissional processwithin the reactor vessel 20. Alternatively, instead of employing theentrainment separators 154 and 172 for respective ones of the drain tankcondensers 1 14 and 115, a flash section can be built into the vaporspace in each drain tank and a suitable entrainment separator (notshown) can be included in this space thereby eliminating the externalseparators 154 and 172.

The storage tanks 52' are provided in addition with a relief header,denoted by the reference character 182. The relief header 182 isconnected to the individual storage tanks 52' by means of conduits 184containing re-. lief valves 185 and coupled respectively to the vaporconduits 146 of each tank 52'. The relief header 182 is coupled to aninput relief header 186 associated with the deuterium oxide storage tankcondenser 188 presently to be described. Likewise, coupled to the inputrelief header 186 by means of suitable connections (not shown) are othercomponents of the reactor system, as indicated by conduit segments 190.In order to satisfy the ASME code there are no valves in the reliefheaders 182 and 186 other than the relief valves 185.

Storage space for the slurry vehicle or in this case heavy water isprovided by a plurality of storage tanks 192, with three being employedin this example. During normal operation of the reactor system thestanks are approximately half full, whereas during a plant shutdown thetanks are completely full. A storage tank condenser 188 is coupled to acommon storage tank outlet conduit 194 through a conduit 196. Any vaporformed in the storage tanks 192 is conducted to the overhead condenser188 by means ofan overhead conduit system 198. The outlet conduit 194 iscoupled to the suctional side of the deuterium oxide pump 200 through avalved conduit 202. When supplying deuterium oxide vehicle to thechemical processing plant 118 for preparation of slurry, makeuppurposes, or the like, the conduit 202 is coupled thereto throughanother valved conduit 204. By means of the pump 200 the vehicle storedwithin the tanks 192, for purposes noted hereinafter, can be supplied tothe primary reactor system through a conduit 206 which is connected tothe associated branched conduit 104 of one of the circulating loops 25.A check valve 208 is coupled in the conduit 206 in order to preventreverse fiow from the primary reactor system or from the high headslurry pumps 122.

14 With this arrangement the primary reactor system can be filledinitially with deuterium oxide vehicle during the processes of startingup and shutting down, presently to be described.

Due to their auxiliary function as catch tanks for relief purposes thedeuterium oxide storage tanks 192 must be limited to a maximum diameterof three feet in this example inasmuch as it is conceivable that thecirculated fuel slurry could be conducted to these tanks 'by way of theinput relief header 186 and storage tank condenser 188. As pointed outpreviously in connection with the drain tanks 52', a tank of thisdiameter is positively incapable of containing a critical mass of theaforesaid fuel slurry. The storage tanks 192 also are employed asaccumulators for liquid or vapor discharged from other components of thereactor plant either normally or during relief situations. Employment ofthese storage tanks 192 as relief volume is desirable inasmuch as theiroperating pressure is relatively low and ranges from about atmosphericto p.s.i.a. The condenser 188 thus serves to limit the maximum pressureduring a relief operation and also to remove radioactive decay heat bycondensation of the attendant vapors, developed within any slurry thatmay be conducted to the deuterium oxide tanks 192.

The initial quantity of deuterium oxide or other slurry vehicle requiredfor the reactor system is added to the storage tanks 192 from a primarystorage system (not shown) by means "of a conduit 210 which joins theoverhead conduit system 198 of the storage tanks 192. Another conduit212 is joined to the outlet side of the deuterium oxide pump 200,whereby a quantity of the denterium oxide is supplied to a suitable gashandling system, such as that noted previously, Where it serves as adiluent for the radioactive gaseous fission products conveyed throughthe latter-mentioned system. The latter portion of deuterium oxide canbe supplied to an external evaporator (not shown) or one [associatedwith the gas handling system, and the deuterium oxide steam not used inthe gas handling system is returned through a conduit 214 to the storagetank condenser 188. The D 0 storage tanks and associated components inaddition are pressurized by the aforesaid evaporator to prevent vaporbinding in the D 0 pump 200.

Slurry concentration and dilution is performed in the drain tank complexor alternatively in the slurry concentrator 220 FIG. 2A, presently to bedescribed. The operations of concentration and dilution are undertakenin order to facilitate starting up and shutting down the primary reactorsystem. Concentration in the drain tank complex is done by boiling andutilizes the radioactive decay heat of the slurry itself. The resultantvapor is condense-d in the drain tank condensers 114 and and iscollected in the deuterium oxide storage tanks 192. In furtherance ofthe latter purpose valves 222 and 224 disposed in the outlet conduits168 and 174, respectively, of the drain tank condenser 114 and 115 areclosed; and the condensate, leaving the drain tank condensers 114 or115, is conducted instead to the input relief header 186 and through thedeuterium oxide storage tank condenser 188 through a valved conduit 230and through conduit 226 or 228, respectively. With this arrangement inutilizing the decay heat as aforesaid the desired slurry concentrationcan be obtained in each group of tanks 5213 and 52C.

As indicated heretofore, the concentration of slurry contained withinthe smaller group of drain tanks 52B is about double that initiallyconveyed from the primary reactor system to the other group of tanks52C. When the concentration of slurry contained within the repositorytanks 52C is increased in the aforesaid manner to that desired for theconcentrated slurry tanks 52'B, for example, the vapor pressuredeveloped in the tanks 52C can be employed to transfer the contents ofthe latter tanks to the other group or groups of tanks 52'B or 52Adepending upon the storage volume required. In furtherance of thispurpose, valves 334 and 336 disposed in the drain and vapor conduits 98cand 1460 of each repository tank 52C are closed, whereupon the vaporresulting from decay heat of the lurry within these tanks tends toaccumulate at the top thereof. At the same time a valve 341 in theextended connecting conduit 112c142 of each tank 52C and the valve 136of the liquid header 110 are opened whereupon the increasing vaporpressure forces slurry from the tanks 52'C into the liquid header 110.At this point the other liquid header valve 134 can be opened; and byopening selected ones of valves 138 and 340 disposed in the connectingconduits 112a and 11211 of the accumulator tank 52A and of theconcentrated slurry tanks 52'B, the slurry forced out of tanks 52'C canbe deposited in one or more of the tanks 52'A and 52B. Alternatively,valves 134 and 342 can be closed, and upon opening valves 320 and 344 inthe conduits 130 and 132, respectively, the

contents of the repository drain tanks 52'C, if relatively small inquantity, can be conveyed directly to the slurry accumulator tank 52'Ain bypassing relationship with the concentrated slurry tanks 52B. Theaccumulator tank 52'A is then isolated, as in normal operatingconditions, from the concentrated slurry tanks 52'B by closing the valve138 situated in the associated connecting conduit 112a. Alternatively,slurry can be transferred among the individual drain tanks 52' of thedrain tank .complex by coupling selected ones of the tanks, from whichmaterial is to be removed, to the liquid header 110 by opening valves134 and 136 when necessary and -'by opening associated ones of valve138, 340 and 341 in the connecting conduits 112a, 1121? and 1120,respectively. The slurry pumps 122 are then energized to draw materialfrom the selected tank or tanks via the liquid header 110 and conduit332 to the drain header 96,

after opening valves 320, 342 and 333. The material then is deposited inselected ones of the tanks 52 through one or more of the conduits 08a,98b, and 98c upon opening appropriate ones of their individual valves335, 337 and 334 respectively, and upon opening appropriate ones of thevalves 100, 102 and 108 in the drain header diluent deuterium oxide can'be diverted to selected groups or individual ones of the drain tanks52' by suitable manipulation of valves 134, 136 and 138. Alternatively,

- liquid D 0 can be removed from the tanks 192 by increasing thepressure of D 0 steam supplied thereto through conduit 214.

In the event that sufficient decay heat is not available forconcentrating the slurry in this fashion, a slurry concentrator 220 iscoupled to the lower manifold 22 (FIG. 1) of the reactional vesselthrough a valved conduit 233 and a second valved conduit 240. Thusduring normal reactor operation a quantity of slurry can be sup plied tothe concentrator 220 upon opening valves 242 and 244 disposedrespectively in the conduits 238 and 240. When, however, the primaryreactor system is not in operation, the slurry can be drawn from thedrain tanks 52' through the connecting conduits 112, the liquid header110, and associated conduits 130 and 128 by means of the high head pumps122. A valve 246 in the outlet conduit 124- of the pumps 122 is closedin order to cause the slurry to flow through a valved connecting conduit248 coupled between the outlet conduit 124 and the inlet conduit 240 ofthe slurry CQ JCGDLLZJLOI 220. Closing the valve 246,

of course, causes the slurry to flow to the concentrator 220 inbypassing relation to the primary reactor system.

The slurry concentrator 220 in this arrangement comprises a battery ofhydroclones (not shown), r the like, which are designed for highpressure operation. The hydroclone arrangement, or other centrifugal orseparating device, separates the slurry into dilute and concentratedfractions, which exit from the concentrator 220 through the conduits 250and 252, respectively. Upon opening valve 251 in conduit 250 and closingvalve 253 in the return conduit 300, the dilute slurry stream isconducted through a suitable cooler 254 to prevent flashing of thedilute and concentrated slurry streams in the depressurizing or letdowndevices, presently to be described. Similarly the concentrated slurrystream is conveyed through cooler 256 by opening valves 255 and 257 inthe conduit 252 and by closing valves 259 and 261 in conduits 296 and298, respectively. The coolers 254 and 256 comprise, for example,ordinary heat exchangers which are designed for the operating pressuresof the reactor system and are cooled by cooling water supplied theretoby means of conduits 258 and 260.

From the cooler 256 the concentrated slurry stream, which is aboutdouble the normal slurry concentration of the primary system, isconveyed through a slurry letdown device indicated generally by thereference character 262. The letdown device 262 consists of relativelysmall diameter tubing desirably inserted in a suitable cooling medium(not shown) to prevent flashing of the concentrated slurry in the eventthat the slurry is not sutliciently cooled in the cooler 256, and havingsufiicient length to induce the desired pressure drop. At this point theconcentrated slurry is added to the drain header 96 and thence to thedrain tanks 52 through a valved conduit 264, after the pressure of theslurry has been reduced by the letdown device 262 from the reactoroperating pressure of 2000 psi. to about 600 p.s.i. or the operatingpressure of the drain tanks 52. A check valve 266 is coupled in thedrain header 96 between the conduit 264 and the remainder of thecomponents connected to the drain header in order to prevent reversefiow from the drain header to the letdown device and associatedcomponents.

A small portion of the slurry concentrated in this fashion isperiodically withdrawn from the outlet stream of the letdown device 262through a valved conduit 267. The latter conduit 267 couples theconcentrated letdown device to a slurry measuring tank 263. With themeasuring tank 268 the amount of concentrated slurry, which periodicallyis extracted from the primary reactor system and is conducted to thechemical processing plant 118 through valved conduit 270, can beproportioned precisely. Any part of the contents of the slurry measuringtank 268 which is not conducted to the chemical processing plant areconveyed to the drain header 96 by means of a valved conduit 272.

The dilute slurry stream issuing from the concentrator 220 is conductedafter passing through the cooler 254 to a depressurizing or letdowndevice indicated generally by the reference character 274. The letdowndevice 274 is similar in structural detail to the concentrated slurryletdown device 262 and accordingly a further description is dispensedwith. A relatively small portion of the output of the letdown device 274in this arrangement is conveyed to an evaporator 276 through a valvedconduit 278. The remainder of the diluted output is conveyed to the D 0storage tank 192 through a valved conduit 322. In the evaporator 276, aportion of the slurry vehicle, or deuterium oxide in this case, isvaporized by means of process steam (H O) supplied to heating coil 280.The remaining slurry, after a portion of the vehicle is removed, isconveyed from the bottom or outlet of the evaporator 276 through avalved conduit 282 to the slurry accumulator tank 52A. The vaporgenerated within the evaporator 276 is conveyed to a purge condenser284. In the purge condenser 284 the vaporous vehicle is conpurgingliquid first is conducted through a cooler 290 in order to prevent vaporbinding in the high head pump 288. A check valve 292 is disposed in aconduit 294 between the cooler 290 and the pump 288 in order to preventreverse fiow in the event of pump failure. The purging system justdescribed in this example serves desirably as an auxiliary purgingsystem for use during reactor shutdown or other contingency. Duringnormal reactor operation, purging liquid is supplied from a fissionalgas handling system usually associated with the reactor plant, such asone of the gas-handling systems disclosed and claimed and theaforementioned copending applications of D. F. Rinald and of J. Weismanet al.

In the event that it is not desired to separate the slurry into lightand heavy fractions by means of the concentrator 220, the concentratorcan be bypassed through the conduits 296 and 252 by opening valves 257and 259 and closing valves 255 and 261. Slurry can then be supplieddirectly from the primary reactor system to the cooler 256 andconcentrated slurry letdown device 262 for depressuriz-ing in the mannerdescribed heretofore and conveyance for instance to the chemicalprocessing plant 118 via the slurry measuring tank 268. The heavy ordilute fractions issuing from the concentrator 220 can be returned tothe primary reactor system, for purposes of starting up or shutting downthe reactor as explained more fully hereinafter, by means of valvedconduits 298 and 300, with the former conduit being coupled to thesuctional side of one of the primary loop pumps 36, The valved conduits298 and 300 are coupled individually to the outlet conduits 250 and 252of the slurry concentrator 220, and a check valve 302 or 304 isconnected in each of the conduits 298 and 300 in order to preventreverse flow from the primary reactor system to the slurry concentrator220 or to the slurry coolers 254 and 256. With this arangement, then,dilute slurry or slurry vehicle can be returned to the primary reactorsystem in shutting down the reactor by closing valves 251 and 261 and byopening valves 253 and 303 in conduits 300 and 298 respectively.Similarly concentrated slurry can be returned, during start-up byopening valves 255, 261 and 303 and by closing valves 253 and 257. Inthis arrangement, the conduit 298 is coupled to the suction side of oneof the circulating pumps 36 to supply the necessary motive force inreturning the dilute or heavy fractions to the primary reactor system.However, an auxiliary pump can be coupled in the conduit 298, ifdesired. The rate of return of either the light or heavy fraction isadjusted by suitable setting of the valves 253 or 261, respectively.Motive force for the light or heavy fraction being depressurized isprovided, of course, by th pressure drop across the letdown device 274or 262.

The normal process of transferring material into or out of the primaryreactor system is accomplished by means of the D storage tank pump 200and the high head slurry pumps 122, in conjunction with the slurryconcentrator 220. The D 0 storage tank pump 200 is provided in orderinitially to fill the primary reactor system during start-up with heavywater at a relatively high rate and low pressure of about 150' p.s.i.a.,while the high head slurry pumps 122 are employed to transferconcentrated slurry from the drain tanks 52B to the primary reactorsystem at a considerably lower rate. The concentrated slurry is added atabout 2000 p.s.i.-a., to which pressure, the initial D 0 fill is raisedby operating the pressurizing vessel 64. At the same time, the slurryWithdrawn from the primary reactor system through the conduit 238 (FIG.2B) is fed through the slurry concentrator 220, and a selected one ofthe light and heavy reactor in the normal manner.

fractions of the slurry concentrator output is returned respectivelythrough the conduits 300 and 298 from the concentrator conduit 250 orthrough the conduit 298 from the other concentrator outlet conduit 252,as the case may be, to the primary reactor system.

As will be described presently in greater detail, the aforementionedlight fraction is fed back into the primary reactor system when it isdesired to dilute gradually the circulated fuel slurry in order toshut-down the At this time the high head pumps 122 are employed to pumpD 0 or very dilute slurry from the storage tanks 192 to the primarysystem through conduits 202, 306, 130, 128 and 120. On the other hand,the aforesaid heavy fraction is fed back to the primary system in orderto increase gradually the concentration of the circulated slurry whenstarting-up the reactor. However, during an emergency the entirecontents of the primary reactor system may be transferred to the draintanks 52 rapidly, in this example in about fifteen minutes, simply byopening the nine pairs of drain valves 106, three pairs of which areshown in FIG. 2B. This emergency drainage procedure is avoided if at allpossible, because of the high thermal stresses and the erosional damageinduced in the drain tank complex and associated components of thesystem, when contacted suddenly by the large volume of heated slurryfrom the primary reactor system. During normal reactor operation thisproblem is not encountered because only relatively small volumes ofslurry are removed from the primary reactor system by means of theslurry concentrator 220 and the dilute and concentrated slurry letdowndevices 274 and 262, respectively.

In the course of normal reactor operation, approximately 47 gallons ofthe circulated slurry are removed daily, in this example, and conveyedto the chemical processing plant 118 for removal of fissional productsand other accumulated impurities. This quantity of slurry is selectedfor processing on the basis that the entire contents of the primaryreactor system, or about 19,000 gallons will be reprocessed over a400-day cycle. As indicated previously, the quantity of slurry thusextracted each day is determined through use of the measuring tank 268.The primary reactor system is replenished with an equal volume of freshslurry from either the concentrated drain tanks 52B or from the slurrypreparation facilities (not shown) of the chemical processing plant 118.

When starting up the aforedescribed reactor system, the primary systemis initially filled by means of a so-called push-pull method. In brief,this procedure consists of filling the reactor vessel 20 and the fourassociated primary circulating loops 25 with a slurry vehicle such asliquid deuterium oxide, supplying an over-pressure by means of thepressurizer 64 described heretofore in connection with FIG. 1 of thedrawings so that the primary pumps 36 can be started, and finally addinga slurry of double the final concentration from the drain tanks 52Bwhile at the same time extracting dilute slurry by means of theconcentrator 220 until the desired operating concentration of thereactor is attained. In the following tables I and II the deuteriumoxide requirements and the capacity of the several systems andsub-systems illustrated in FIG. 2 of the drawings are given:

TABLE 1 D 0 requirements IL: TABLE II Various system and sub-systemcapacities Gal. Primary reactor system 19,100 D storage tanks (192)21,400

Drain or repository side of drain tank complex (SZC) 19,100

Concentrated slurry side of drain tank complex (52'A and 52B) 10,300Auxiliary equipment 2,000

From the preceding tables, it will be seen that the initial deuteriumoxide requirement totals 28,650 gallons, in this exemplary arrangement.The heavy water is fed int-o the D 0 storage tanks 192 from which 5,650gallons are withdrawn to four of the slurry repository tanks 52C bygravity flow through the .outlet conduits 194- and 202 of the D 0storage tanks 192 and a valved conduit 306 coupling the last-mentionedconduit 202 to the liquid header 110 of the drain tank complex. A checkvalve 308 is disposed in the connecting conduit 306 to prevent reverseflow through the conduit 306 from the liquid header 110 and the draintank complex. The aforementioned 5,650 gallons are withdrawn, of course,as the D 0 storage tanks 192 are being filled, inasmuch as the totalvolume of the storage tanks 192 in this example, is less than theinitial D 0 requirements. An additional 1,600 gallons are sent by meansof the D 0 storage pump 200 and conduit 212 to the evaporator (notshown) emplayed for example in one of the aforementioned gas handlingsystems. The latter quantity of D 0 initially supplies operating purgingliquid until the gas handling system attains normal operatingconditions.

Prior to adding slurry to the reactor system, it is necessary to obtaina supply of purging water, as aforesaid. As an alternative supply ofpurging Water, a portion of the heavy water contained within the storagetanks 192 is transferred by gravity to the evaporator 276 describedheretofore by means of a valved conduit 310. In the evaporator 276 theheavy water is vaporized and subsequently condensed in the purgecondenser 284 as described heretofore in connection with the diluteslurry fraction vehicle. The D 0 withdrawn from the storage tanks 192,is conducted first to the evaporator 276 in order to remove from thepurge water any slurry particles which may have accumulated in thestorage tanks. From the condenser 284 the heavy water is conductedthrough the cooler 290 and supplied to the suction side of the high headpump 288 for purging purposes. From the pump 288 the condensed purgeliquid is carried to a purge storage tank 312 through an outlet conduit314. In the purge storage tank 312 a pressure in excess of the reactoroperating pressure of 2,000 p.s.i.a. is maintained by means of a helium.or another inert gaseous atmosphere supplied from a suitablepressurized source (not shown). At this point, high pressure water isnow available to a purge header 316 which is coupled to those componentsof the reactor system which require purging, for example, the pumps andthe entrainment separators as indicated by the partial conduits 286.

After a supply of purging water is made available, approximately 7,250gallons of heavy water are transferred to the chemical processing plant118 for preparing the concentrated slurry which is employed as explainedhereinafter for filling the primary reactor system. In order to obtainthe last-mentioned quantity of heavy water, the 5,650 gallons alreadypresent in four of the drain tanks 52'C plus the steam condensed duringpressurization of the D 0 storage tanks and the drain tanks aresupplemented by heavy water from the D 0 storage tanks 192. Thepressurizing steam is supplied to the drain tanks 52' from the drainheader 96 via conduit 317, and to the storage tanks 192 through theconduit 214 from a suitable evaporator (not shown). However, at least19,100 gallons must be left in the storage tanks 192 for initiallyfilling the primary reactor system with the slurry vehicle. The slurryis then returned from the processing plant 118 by means ,of the transferpump 318 coupled in the outlet conduit 121 of the slurry or chemicalprocessing plant. From the pump 31%, the concentrated slurry isconducted to the liquid header through the conduits 128 and 130 afteropening valves 320 and 342. The valve 136 of the liquid header is thenclosed and the valve 134 thereof opened with the result that theconcentrated slurry is conducted into the concentrated slurry tanks SZBthrough the individual conduits 1123. At this time the slurryaccumulator tank SZA is isolated from the liquid header 110 by closingits associated valve 138.

The primary reactor system including the reactor 20 and associatedcirculating loops 25 is now filled with the 19,100 gallons of deuteriumoxide or other slurry vehicle contained in the D 0 storage tanks 192, bymeans of the D 0 storage pump 200. Subsequently, the pressurizing vesselis charged with deuterium oxide from a high pressure D 0 storage tank(not shown) via conduit 321. The heaters 70 of the pressurizing vessel64 are then energized to induce an initial pressure of several hundredpounds per square inch within the primary reactor system or a pressuresufficient to prevent vapor binding in the circulating pumps 36. Thefull system pressure of 2,000 p.s.i.a. should not be applied to theprimary system until a temperature of at least 200 F. is attained inorder to avoid the tendency of the structural steels to fracture due tobrittleness at relatively low temperatures. For the same reason, noslurry or circulating fuel is permitted in the primary reactor systemuntil the aforesaid 200 F. temperature level is reached. This precludesthe possibility of a sudden pressure surge or positive system transientbefore the minimum temperature mentioned above is attained.

Accordingly, it is necessary to supply external heat to the primaryreactor system before adding any slurry thereto inasmuch as the initialreactor filling of deuterium oxide is added substantially at roomtemperature. In order to avoid excessive thermal stresses in the heavyWalls of the reactional vessel 20, a heating rate of 50 F. per hour isemployed. One arrangement for supplying the necessary heat consists offilling the reactor system with relatively cold deuterium oxide or otherslurry vehicle, operating the primary pumps 36, and thus adding heat aspump work. Alternatively, or in conjunction therewith, heat can be addedto the primary system through the steam generators 32 by means ofprocess (H O) steam supplied to the steam side of one or more of thesteam generators from an external boiler arrangement (not shown).

Before slurry can be introduced into the primary reactor system from theconcentrated slurry drain tanks 52'B, the repository drain tanks 52Cmust be prepared as a safety precaution, for the possibility of anemergency drain. In furtherance of this purpose the tanks of 52'C aremaintained at 250 F. or greater at all times. This is accomplished byemptying the drain tanks 52C and continuously heating them with 100p.s.i.a. D 0 steam supplied to these tanks through the conduit 317 (FIG.213) from an external evaporator, as aforesaid. To heat the tanks 52'Cfrom room temperature or 60 F. to 250 F. requires the heat equivalent ofapproximately 15,000 pounds of D 0 vapor and the capacity of theexternal evaporator is such that this amount can be generated in fifteenminutes. However, when thermal equilibrium is once obtained, D 0 vaporis condensed at the rate of only pounds per hour in maintaining atemperature of 250 F. in the entire group of the thirteen repositorytanks 52C. Additionally, the tanks SZC should be isolated from otherportions of the reactor system so that these other portions will not besubjected to thermal stresses induced by drainage of hot slurry from theprimary reactor system into the repository tanks 52'C. Moreover, thedrain tanks 52B should likewise be.

21 isolated at this time to avoid diluting the concentrated slurrycontained therein.

Slurry is now transferred from the concentrated slurry tanks 52'B to theprimary reactor system by means of the high head pumps 122. As theconcentrated slurry is added to the primary reactor system, anequivalent amount of the initial deuterium oxide filling is removed fromthe primary system and this is continued until the desired slurryconcentration is obtained. Removal of the initial deuterium oxidefilling is accomplished by means of the slurry concentrator 220 which isdesigned to withstand an operating pressure in the neighborhood of 2,500p.s.ia. slurry is added to the primary reactor system from the draintanks 52'B through conduits 112B, the liquid header 110, the high headpumps 122, and the conduit 124, a similar quantity of liquid isextracted from the reactor system by opening valves 242 and 244 in theconduits 238 and 240, respectively, whereupon a quantity of the initialfilling is conducted to the slurry concentrator 220. The heavy orconcentrated fraction of the slurry concentrator output is returned tothe suctional side of one of the primary pumps 36 through the conduits252 and 298 upon opening appropriate valves therein. The light or diluteoutput of the slurry concentrator 220 at this time is conducted to thecooler 254 and the dilute slurry letdown device 274, and thence isreturned to the D storage tanks 192 through the storage tank conduit 198and a valved conduit 322. It will be seen from this arrangement that noadditional pump is required for conveying fluid from the primary systemto the slurry concentrator 220 inasmuch as one of the primary pumps 36supplies the driving head.

For a normal shutdown operation the flow paths of the light and heavyfractions issuing from the slurry concentrator 220 are reversed suchthat the heavy fraction is conveyed to the drain tanks 52 by means ofthe cooler 256, the concentrated slurry letdown device 262, and thedrain header 96. On the other hand, the light fraction of the slurryconcentrator 22 is conveyed through conduits 250, 300 and 298 to thesuctional side of one of the primary pumps 36. In this manner, theconcentration of the slurry circulating through the primary reactorsystem is diluted gradually until the reactor becomes sub-critical. Ofcourse as slurry is conveyed to the slurry concentrator 220, during theshutting down procedure, an equivalent amount of' deuterium oxide orother vehicle is added as a diluent to the primary system from the D 0storage tanks 192 by means of conduits 202, 306, 130, 128 and 120, andthe high head pumps 122.

During start-up of the reactor, slurry is fed to the primary reactorsystem at a concentration of approximately 600 grams of thorium oxideand uranium oxide per kilogram of D 0 and at a rate of 30 gallons perminute or less until criticality is reached. During the time in whichconcentrated slurry is added in this fashion the liquid level in thepressurizing vessel 64 is main tained constant by controlling the rateof diluted slurry passing through the letdown device 274. The averagetemperature in the primary system during this portion of the fillingoperation has risen to approximately 300 F. so that criticality isobtained in the reactor vessel 20 at an average slurry concentration ofabout 40 grams per kilogram of D 0 as shown at point 327 on curve 338 inFIG. 3 of the drawings. At this point of course, the eifectivecoefiicient of critically (k is equal to unity. The slurry mustthereafter be introduced at a reduced rate not only to avoid largeinputs of reactivity to the reactor system but also to limit the rate oftemperature rise to 50 F. per hour and to maintain (k l). The additionof concentrated slurry is continued until the ope-rating temperature of522 F., at substantially zero power level is reached. This point isdesignated as the .low critical concentration and occurs in thisarrange- More specifically, as the concentrated ment at a slurryconcentration of about 70 grams per kilogram of D 0 as shown by point319 on curve 329.

At the aforedescribed low critical concentration the re actor desirablyis operated at a substantial percentage of rated power output inaccordance with the method described and claimed in the aforesaidcopending application of W. A. Frederick as pointed out previously.Alternatively, an external reactor poison can be added at this time tocause the reactor to become subcritical while the concentration of thefuel is being increased to the high critical concentration. On the otherhand, the use of the aforementioned poison can be eliminated altogetherand the reactor system can be designed to withstand higher peaktemperatures and attendant pressures so that the temperature of thecirculating slurry can be permitted to rise sufliciently to shut downthe reactor at the low critical concentration before adding additionalfuel to the slurry. This is accomplished for example by withdrawing onlyenough steam from the steam generators 32 by suitably adjusting thevalves 324 in their outlet conduits 37 so that the average temperaturewithin the reactional vessel 20 reaches at least 595 at whichtemperature the reactor will remain subcritical, in the arrangementdescribed herein for that range of concentrations between the high andlow critical points as indicated by peak 330 of temperature curve 360illustrated in FIG. 3 of the drawings. It will also be seen from FIG. 3that this maximum temperature is obtained as the concentration isincreased from the low critical concentration of 70 grams per kilogramof deuterium oxide to only about 170 grams per kilogram of deuteriumoxide. Thus, by removing only a small proportion of power in the form ofheat from the primary system a resultant temperature rise prevents thereactor from becoming critical as the c0ncentration is increased. Theresultant expansion of the D 0 vehicle-moderator is illustrated in thefollowing Table HI, which expansion results in a negative coeflicient ofreactivity as explained previously:

TABLE III D 0 volume expansion due to temperature increase Percent 60 F.to F. 1.8 60 F. to 300 F. 9.3 60 F. to 522 F. 33.8 120 F. to 522 F. 31.4

300 F. to 522 F. 22.8

As soon as the reactor becomes subcritical, heat removal via the steamgenerators 32 is determined substantially to aid in preventing thereactor from again reaching criticality while the concentration of thefuel is being increased to the operating or high critical concentration.If the reactor is operated for a time at the low critical point, thelimited amount of subsequent fissions plus the decay heat involved byfissional products generated during the period of criticality willmaintain the temperature of the reactor system at or above theaforementioned temperature of 595 until the concentration can beincreased to the high critical concentration.

After the average reactor temperature of 522 F. has been obtained, whichis the operating temperature in this case, the temperature is thenmaintained constant by extracting power in the form of heat from theprimary reactor system by means of the steam generators 32. Withdrawingpower, of course, increases the temperature drop across the reactionalvessel 20, as illustrated in FIG. 4 of the drawings, while the averagetemperature of the vessel remains constant as shown by line 346.However, the slurry temperature at the vessel outlet manifold 24(FIG. 1) rises linearly, line 348, with power level until approximately580 F. is attained at full power operation; and the slurry is suppliedto the steam generators 32 at substantially the outlet manifoldtemperature. On the other hand, the slurry temperature at the inletmanifold 22 (FIG. 1) decreases linearly with increased reactor poweroutput, line 350, to about 465 F. at full reactor power. The temperatureof steam issuing from the steam generators 32 decreases linearly withthe quantity of steam utilized as shown by line 352; thus, thetemperature of the steam likewise decreases linearly with increasedreactor power, to about 445 F. at rated power output.

The process of shutting down the reactor system under normal conditions,for example during inspection or maintenance of equipment, requiresessentially the reverse of the steps employed in the aforedescribedstarting up procedure. Initially the heat withdrawn from the reactor isreduced substantially so that the resulting increase of temperature willrender the reactor subcritical. The reactor then is maintained in asubcritical condition by one of the methods outlined previously whilethe slurry concentration is decreased from the high criticalconcentration to the low concentration.

The slurry concentration is reduced by pumping in D vehicle at the rateof 30 gallons per minute and by utilizing the slurry concentrator 220 toreturn the light or dilute slurry fraction to the primary reactor systemthrough conduits 300 and 298 and to conduct the heavy or concentratedfraction through the slurry letdown device 262 for storage in the draintank complex. In furtherance of this purpose, dilute slurry or slurryvehicle, as the case may be, from the D 0 storage tanks 192 is fed intothe primary reactor system by means of the high head pumps 122 andassociated conduits. The concentration of slurry within the primarysystem should be reduced as far as practical in order that flushing ofthe primary system is not required subsequent to the normal drainingprocedure. Obviously, the dilution process can be terminated orsuspended temporarily at the low critical concentration if it is desiredto operate the reactor to sustain a chain reaction at the low criticalconcentration. This latter procedure is efiicaceous, for example, inmaximizing burn-up in a given quantity of slurry, in obtaining fissionalproducts for industrial purposes, or the like. Following operation atthe low critical concentration, if used, the chain reaction can beterminated simply by reducing the concentration of the slurry, asindicated by the isothermal curves of FIG. 3.

The slurry initially removed from the primary system by the slurryconcentrator 220 is approximately double the normal concentration and asthe concentration in the primary system decreases so will that of theconcentrated output of the slurry concentrator 220. The heavy fractionof the slurry concentrator output is stored in the concentrated slurrytanks 52'B to which the heavy fraction is conveyed by means of conduits252, 264, and 96 and associated components as described previously.However, after the heavy fraction of the slurry concentrator 220 hasdecreased substantially below double the normal reactor concentration,the slurry then is reconcentrated by conducting it from the drain tanks52'B to the concentrator 220 and returning only the heavy fraction tothe drain tanks 52'B. This is accomplished by conveying the slurrythrough the liquid header 119, conduits 130 and 128, pump bypassingconduit 354, conduit 124, and primary reactor system bypassing conduit248 to the slurry concentrator inlet conduit 240. Then the heavyfraction is returned via the concentrated slurry letdown device 262. Thenecessary driving force is furnished by heat of radioactive decay, or ifthe latter is insufiicient by an over pressure supplied from theaforesaid external evaporation through conduits 317.

Alternatively, inasmuch as release of radioactive decay heat within theslurry stored in the drain tanks 52B will result in removal of excessslurry vehicle by vaporization, the slurry can be permitted to increaseto the desired consistency without removal from the drain tanks 52B. Thevapors thus removed from the drain tanks are condensed in the drain tankcondensers 114 and 115 as described heretofore and the resultant liquiddesirably is 2d conveyed through the conduits 226, 228 and 230 to the D0 storage tanks 192. After the slurry has been concentrated toapproximately double the normal concentration within the drain tanks 52Band the concentration of slurry within the primary system has beenreduced to a corresponding concentration of about 30 grams per kilogramof D 0, the pairs of drain valves 106 are opened in the conduits 54 and104, and the slurry is conveyed to the repository tanks 52C via thedrain header 96 and associated conduits. This low concentration isselected in order to reduce erosion of the drain valves 106 insofar aspractical. More particularly, this concentration is selected because theslurry cannot be criticalized at any temperature at this concentration,as would be indicated by extrapolation of the temperature curves of FIG.3.

During the shutdown procedure decay heat is removed at a lowercontrolled rate from the primary system during the slurry dilutionprocess by reducing the water level at the steam side of the steamgenerators 32. Preheated feed water is employed for this purpose as inthe case of starting up the reactor to prevent thermal shock, and theresultant steam is condensed in the turbine condenser (not shown) or thelike. When the slurry concentration has been reduced in the primarysystem such that the average system temp erature is in the neighborhoodof 250 F., heat removal by means of the steam generators should beterminated in order that the primary system will be maintained at thistemperature by the aforesaid heat of radioactive decay. At thistemperature the reactor can be restarted conveniently without imminentdanger of thermal shock. Moreover, the drain tank complex desirably ismaintained at this temperature, as described heretofore in connectionwith the start-up procedure, so that in the event of emergency drain theheated slurry conducted to the drain tanks 52C will not subject thesetanks to undue thermal stresses.

When the slurry concentration has been reduced to a sufilciently lowlevel that substantially no slurry particles will settle out in theprimary system, the primary pumps 36 are shut down, the heaters 70 ofthe pressurizing vessel 64 are deenergized, and the nine pairs of drainvalves 106 are opened, whereupon the contents of the primary system areemptied into the drain tanks 52C. In this manner, thermal shock in thedrain tank complex will be practically nil and erosional damage to thedrain valves 106 will be minimized or eliminated altogether. Thepressure in the drain tanks 52 resulting from decay heat is controlledby varying the cooling water flow to the drain tank condensers 114 and115. The maximum pressure of the drain tanks, is arbitrarily set at 600p.s.i.a. although the tanks are designed for 1500 p.s.i. This safetyfactor is desirable inasmuch as available cooling water may beinadequate during an emergency drain.

As indicated heretofore, the slurry concentration in each group of draintanks SZ'B and 52C is controlled by simply changing the fiow path of thecondensed D 0 vapor from the drain tank condensers 114 and 115. Infurtherance of this purpose, a suitable concentration measuring device(not shown) is associated with each of the drain tanks 52'.

When restarting the reactor the very dilute slurry contained in thedrain tank 52C is forced back into the primary reactor system bysuitable means. The very dilute slurry is withdrawn from the repositorydrain tanks 52C through the liquid header 110 and associated connectingconduits 112a and thence through conduits 130, 128 and 124 to one of thecirculating loops 25. In this example, the high head pumps 122 are ofrelatively low volumetric capacity, and therefore, are not employed forinitially filling the reactional vessel 20 and associated circulatingloops 25. For this reason, the high head pumps are bypassed with avalved conduit 354, inasmuch as the initial reactor filling of verydilute slurry is incapable of sup- 25 porting a chain reaction and canbe introduced at a rapid rate to save time in the restarting procedure.

In one arrangement the driving force required to transfer the diluteslurry from the drain tank complex to the primary reactor system issupplied by means of an overpressure applied in the form of D steam fromthe aforementioned external evaporator. The pressurizing D 0 steam isconveyed to the drain tanks 52'C via the conduit 317, the drain header96, and associated connecting conduits 980, after opening appropriatevalves. The increased steam pressure, of course, forces the liquidcontained within the drain tank 52'C through the conduits 112e, whichextend as aforesaid to bottoms of the tanks, and into the liquid header110.

In another arrangement the driving force is supplied by heat ofradioactive decay of the slurry contained within the concentrated slurrytanks 52'B. Since such material usually is relatively concentrated andcontains, after a period of circulation through the reactional vessel20, a quantity of fissional products, a considerable amount of D 0 vaporis evolved from the tanks 52'B by the decay heat of these products. Toemploy this vapor to force the contents of the repository drain tanks52'C back into the primary reactor system, valves 356 and 358 are closedin the conduits 152 and 171 connecting the entrainment separators 154and 172 and drain tank condensers 114 and 115, respectively, with thevapor header 144. At the same time, the valves 148 of the vapor headerare opened, and likewise valve 150, if there is an appreciable quantityof slurry in the slurry accumulator tank 52'A tanks. The vapor issuingfrom 52'B, and 52'A if any, is then conveyed through the vapor header144 to the repository drain tanks 52'C, via their individual connectingconduits 146a after opening valves 336 therein. In this manner the vaporpressure built up in the concentrated slurry tanks 52'B is applied tothe top surfaces of the slurry contained within the dilute or repositorydrain tanks 52'C. As a result the dilute slurry is forced downwardly andout of the drain tank 52'C through the connecting conduits 112c andtheir extensions 142 to the liquid header 110. The valve 134 in theliquid header having been closed, the dilute slurry then is caused toflow through the conduits 130 and 128, the pump bypassing conduit 354,and the conduit 124 to the primary reactor system. Inasmuch as theconcentration of the slurry contained within the repository tanks 52'Cis in capable of sustaining a chain reaction, the primary system can becompletely filled with this dilute slurry as an initial step inrestarting the reactor. Thereafter, the concentration of the slurry isincreased in the manner described heretofore in connection withinitially starting the reactor. This arrangement also leaves the tanks52'C empty in the event of any contingency or emergency during startingup or subsequent operation of the reactor.

An emergency drain procedure is followed in the event of equipmentleakage or failure in the primary reactor system. A rapid drainage ofthe reactor system obviates such conditions as extensive contaminationof the vapor container (not shown) surrounding the reactor plant,considerable loss of slurry and heavy water or other slurry vehicle, andcaking of slurry in the primary system due to failure of the primarypumps or the like. As shown in FIG. 2 of the drawings the primaryreactor system has a total of nine drain connections with one connection50 being made at the lower or inlet manifold of the reactor vessel 20(FIG. 1) and two connections being made at each of the circulating loops25. As described heretofore, each of these connections is coupledthrough conduits 54 or 104 in each of which are disposed a pair of stopvalves 106. This drainage system permits complete drainage from thelowest points of the entire primary reactor system. In this example, theentire contents of the primary system can be drained in approximatelyfifteen minutes.

if a maximum of radioactive decay heat is produced when the slurry isdrained in this fashion to the drain tanks 52, heat will be generatedtherein for a limited length of time at a greater rate than it can beremoved by the drain tank condensers 114 and 115. During the emergencydrain and for a short period thereafter, the temperature and pressure ofthe slurry in the drain tanks will rise to a maximum of about 545 F. and1000 p.s.i.a., in this arrangement.

The drain valves 106 can be operated manually, if desired; oralternatively, these valves can be operated automatically by suitablemechanisms (not shown), which mechanisms are actuated in turn by ahazardous abnormal condition such as an excessively high pressure in theprimary system, detection of radiation in the steam leaving one of thesteam generators 32, loss of power to the primary circulating pumps 36,leakage from the primary gelactor system to the aforesaid vaporcontainer, or the From the foregoing description it will be apparentthat a novel and efficient reactor system has been described herein.Although the invention has been described in connection with aparticular type of homogeneous reactor, it will be apparent that theinvention can be adapted readily to other types of homogeneous reactorsand related reactor systems. The foregoing descriptive and illustrativematerials therefore are intended to exemplify the invention and shouldnot be interpreted as being limitative thereof. Accordingly, numerousembodiments of the invention will occur to those skilled in the artwithout departing from the spirit and scope of the invention.

I claim as my invention:

1. A method for operating a neutronic reactor system capable ofcontaining a fuel material of variable concentration and having high andlow critical fuel concentrations, said fuel material being fluidized ina moderating vehicle therefor, said method comprising the steps ofinitially filling said reactor system with a quantity of said vehiclecontaining at most fuel material at a concentration below said lowcritical fuel concentration, adding a quantity of said fuel material tothe vehicle contained within said reactor system, bleeding a quantity ofsaid vehicle including the fuel material contained therein from saidreactor system and separating said bled quantity into dilute andconcentrated fractions of said fuel material in said vehicle, returningsaid concentrated fraction to said reactor system and continuing theaddition of said fuel material to the vehicle and fuel materialcontained in said reactor system until said low critical fuelconcentration is attained within said system and until said reactorsystem begins to operate, shutting down said reactor system, and furthercontinuing the additions of said concentrated fraction and said fuelmaterial to said reactor system while the latter is maintained in theshut down condition until said high critical fuel concentration isattained within said reactor system.

2. A method for operating a neutronic reactor system capable ofcontaining a fuel material of variable concentration and having high andlow critical fuel concentrations and a negative temperature coefficient,said fuel material being fluidized in a moderating vehicle therefor,said method comprising the steps of initially filling said reactorsystem with a quantity of said vehicle containing at most fuel materialat a concentration below said low critical fuel concentration, adding aquantity of said fuel material to the vehicle contained in said reactorsystem, bleeding a portion of said vehicle including the fuel materialcontained therein from said reactor system and separating said bledportion into dilute and concentrated fractions of said fuel material insaid vehicle, returning said concentrated fraction to said reactorsystem and continuing the addition of said fuel material to the vehicleand fuel material contained in said reactor system until said lowcritical fuel concentration is attained within said system, operatingsaid reactor system at said low critical fuel concentration for a periodsufficient to build up a substantial quantity of fissional products,increasing the temperature of said system sufficiently to shut down saidreactor at said low critical fuel concentration following said operatingperiod, retaining within said system a substantial portion of the heatof radioactive decay of said fissional products while said reactor is inthe shutdown condition to maintain said reactor system in a subcriticalstate until said high critical fuel concentration can be attained, andfurther continuing the additions of said fuel material and saidconcentrated fraction to said reactor system until said high criticalfuel concentration is attained within said system.

3. A method for operating a neutronic reactor system capable ofcontaining a fuel material of variable concentration and having high andlow critical fuel concentrations and a negative temperature coefficient,said fuel material being fluidized in a moderating vehicle therefor,said method comprising the steps of initially filling said reactorsystem with a quantity of said vehicle containing at most a quantity ofsaid fuel material below said low critical fuel concentration, addingfuel material to the vehicle contained in said reactor system until saidlow critical fuel concentration is attained therein, operating saidreactor system at said low critical fuel concentration for a periodsufiicient to build up a substantial quantity of fissional products,increasing the temperature of said system sufiiciently to shut down saidreactor system at said low critical fuel concentration following saidoperating period, retaining Within said reactor system at least asubstantial portion of the heat of radioactive decay of said fissionalproducts while said reactor is in the shut down condition to maintainsaid reactor system in a subcritical state until said high critical fuelconcentration can be attained, and continuing the addition of said fuelmaterial to the vehicle and the fuel material contained within saidreactor system until said high critical fuel concentration is attained.

4. A method for shutting down a neutronic reactor system capable ofcontaining a fuel material of variable concentration and having high andlow critical fuel concentrations and a negative temperature coefficient,said fuel material being fluidized in a moderating vehicle therefor,said method comprising the steps of operating said reactor system atsaid high critical fuel concentration for a period sufficient to buildup a substantial quantity of fissional products, increasing thetemperature of said system sufficiently to shut down said reactor systemat said high critical fuel concentration following said operatingperiod, bleeding a quantity of the vehicle including the fuel materialcontained therein from said reactor system and separating said bledquantity into dilute and concentrated fractions of said fuel material insaid vehicle, adding to said reactor system a quantity of said vehiclecontaining at most fuel material at a concentration less than the lowestfuel concentration at which said reactor system can attain criticality,returning said dilute fraction to said reactor system, continuing theaddition of said vehicle to said reactor system in an amount equivalentto that of said concentrated fraction until the concentration of fuelmaterial in the vehicle within said reactor system is decreased belowsaid lowest fuel concentration, and retaining within said reactor systemat least a substantial portion of the heat of radioactive decay of saidfissional products While said reactor is in the shut down condition tomaintain said reactor system in a subcritical state until said lowcritical fuel concentration can be attained.

OTHER REFERENCES Briggs et al.: Proceedings of the First GenevaConference, 1955, vol. 3, pp. 175187, published by UN.

CARL D. QUARFORTH, Primary Examiner.

1. A METHOD FOR OPERATING A NEUTRONIC REACTOR SYSTEM CAPABLE OFCONTAINING A FUEL MATERIAL OF VARIABLE CONCENTRATION AND HAVING HIGH ANDLOW CRITICAL FUEL CONCENTRATIONS, SAID FUEL MATERIAL BEING FLUIDIZED INA MODERATING VEHICLE THEREFOR, SAID METHOD COMPRISING THE STEPS OFINITIALLY FILLING SAID REACTOR SYSTEM WITH A QUANTITY OF SAID VEHICLECONTAINING AT MOST FUEL MATERIAL AT A CONCENTRATION BELOW SAID LOWCRITICAL FUEL CONCENTRATION, ADDING A QUANTITY OF SAID FUEL MATERIAL TOTHE VEHICLE CONTAINED WITHIN SAID REACTOR SYSTEM, BLEEDING A QUANTITY OFSAID VEHICLE IN SAID REACTOR SYSTEM, BLEEDING A QUANTITY OF SAID VEHICLEINCLUDING THE FUEL MATERIAL CONTAINED THEREIN FROM SAID REACTOR SYSTEMAND SEPARATING SAID BLED QUANTITY INTO DILUTE AND CONCENTRATED FRACTIONSOF SAID FUEL MATERIAL IN SAID VEHICLE, RETURNING SAID CONCENTRATEDFRACTION TO SAID REACTOR SYSTEM AND CONTINUING THE ADDITION OF SAID FUELMATERIAL TO THE VEHICLE AND FUEL MATERIAL CONTAINED IN SAID REACTORSYSTEM UNTIL SAID LOW CRITICAL FUEL CONCENTRATION IS ATTAINED WITHINSAID SYSTEM AND UNTIL SAID REACTOR SYSTEM BEGINS TO OPERATE, SHUTTINGDOWN SAID REACTOR SYSTEM AND FURTHER CONTINUING THE ADDITIONS OF SAIDCONCENTRATED FRACTION AND SAID FUEL MATERIAL TO SAID REACTOR SYSTEMWHILE THE LATTER IS MAINTAINED IN THE SHUT DOWN CONDITION UNTIL SAIDHIGH CRITICAL FUEL CONCENTRATION IS ATTAINED WITHIN SAID REACTOR SYSTEM.